Adjust the selector parameter on your config file. This is usually targets generally selector: "textarea.texteditor",. To limit it to album descriptions targetting selector: "#form_albumedit textarea.texteditor", should work.

Then select your custom config file and it should be limited to the album desc.

Other options to speed up is to choose a slim configuration for the gallery and/or limit the images per page to fewer.

And before you mention it, probably having selectors for images and albums individually would perhaps also be an idea ;-)

The general instructions are just a bit confusing in one aspect. It says "Rename your copied config file(s) to something unique for each file, e.g. add a prefix like "my_" or anything else you wish to use." -- But the new config doesn't show up in the drop-down menu if you add a prefix to the file name, i.e.: prefix_zenphoto-slim.js.php.

It does work though, if you replace "slim" by something individual, like: zenphoto-myconfig.js.php.